Heat effect on FR4?
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From: Uwe Hercksen on 11 Jun 2010 10:04 ehsjr schrieb: > I have a resistance circuit that dissipates ~1.2 watts. If I use > 11 one watt resistors in series (physically parallel) I can spread > the heat out across the width of the board. Hello, why do you want to use 11 one watt resistors to spread about 1.2 watts? If it is only 1.2 W, two or three resistors will do, if it is 12 W, 11 of those resistors will not be enough. There are resistors for 4, 5 and 11 W to be soldered in a FR4 board too. There are also resistors for chasis mount available for 25 or 50 W. If you think about 10 years of 24 x7 operation, you should chose a robust solution. Bye
From: mook johnson on 11 Jun 2010 18:41 "ehsjr" <ehsjr(a)nospamverizon.net> wrote in message news:husj1c$g5l$1(a)news.eternal-september.org... >I have a resistance circuit that dissipates ~1.2 watts. If I use > 11 one watt resistors in series (physically parallel) I can spread > the heat out across the width of the board. I measured ~66C under > each resistor at worst case ambient (~32C) in the test setup. > > What will happen to to the board over time (>10 years) of 24x7 operation? > (Resistors in contact with the board.) Does FR4 behave > like wood that is exposed to temperature over time, slowly charring? > > TIA, > Ed Is that out on the bench outside fo the intended enclosure? If you it will be much hotter when you enclose it.
From: Phil Allison on 11 Jun 2010 20:06 "ehsjr" > >I have a resistance circuit that dissipates ~1.2 watts. If I use > 11 one watt resistors in series (physically parallel) I can spread > the heat out across the width of the board. I measured ~66C under > each resistor at worst case ambient (~32C) in the test setup. ** So that is 110 mW dissipation per 1W resistor or 11% of rated. > What will happen to to the board over time (>10 years) of 24x7 operation? > (Resistors in contact with the board.) ** Probably nothing. I see old equipment all the time with resistors laying flat on the PCB that dissipate more than 25% of rated and there is not even a mark left on the PCB. You could space the body up a few mm from the surface for even greater immunity. ..... Phil
From: ehsjr on 13 Jun 2010 23:52
Phil Allison wrote: > "ehsjr" > >>I have a resistance circuit that dissipates ~1.2 watts. If I use >>11 one watt resistors in series (physically parallel) I can spread >>the heat out across the width of the board. I measured ~66C under >>each resistor at worst case ambient (~32C) in the test setup. > > > > ** So that is 110 mW dissipation per 1W resistor or 11% of rated. > > > >>What will happen to to the board over time (>10 years) of 24x7 operation? >>(Resistors in contact with the board.) > > > ** Probably nothing. > > I see old equipment all the time with resistors laying flat on the PCB that > dissipate more than 25% of rated and there is not even a mark left on the > PCB. > > You could space the body up a few mm from the surface for even greater > immunity. > > > .... Phil > > > Thanks, Phil. I was hoping someone would answer based on experience with the long term effects, so your answer is particularily appreciated. Ed |